Geometrically-spectrally selective radiator



1966 A. L. JOHNSON, JR

GEOMETRICALLY-SPECTRALLY SELECTIVE RADIATOR 5' Sheets-Sheet 1 Filed 001- 17, 1962 INVENTOR: ALFRED L. JOHN$0N,JK BY Atfomey.

Nov. 15, 1966 A. L. JOHNSON, JR 3,285,333

GEOMETRICALLY-SPECTRALLY SELECTIVE RADIATOR Filed Oct 17, 1962 5 Sheets-Sheet 2 I N VENTOR.

A florney.

ALFRED L. JOHNSOMJI:

Nov. 15, 1966 A. JOHNSON, JR

GEOMETRICALLY-SPECTRALLY SELECTIVE RADIATOR 5 Sheets-Sheet 3 Filed OCT, 17, 1962 INVENTORI AL FRED L. uofi/vso/vw A fforney.

United States Patent 3,285,333 GEOMETRICALLY-SPECTRALLY SELECTIVE RADIATOR Alfred L. Johnson, Jr., Hermosa Beach, Calif., assignor to The Garrett Corporation, Los Angeles, Calif., a corporation of California I Filed Oct. 17, 1962, Ser. No. 231,210 8 Claims. (Cl. 165-- 133) This invention relates generally to heat exchange apparatus and particularly to a heat exchanger which is both geometrically .and spectrally selective with respect to its thermal environment.

The heat exchanger of this invention is designed primarily to transfer or dissipate heat from a heat source to the thermal environment in which the heat exchanger operates; that is to say, the present heat exchanger is designed to function primarily as a thermal radiator. F or this reason, the invention will be described main-1y in connection with its use as a thermal radiator. It will become evident as the description proceeds, however, that the heat exchanger of the invention may also be advantageously employed as a geometrically and spectrally selective heat absorber. Acordingly, the invention should not be regarded as limited in application to a thermal radiator.

During operation, many systems generate heat which must be continuously transferred from the systems to maintain the latter at optimum operating temperature. One common method of effecting such thermal control involves transfer of the excess thermal energy from the system to a thermal radiator having external heat exchange surfaces from which the thermal energy is transmitted to the ambient environment.

Depending upon the thermal and physical nature of this environment, such heat transmission may involve any of the three modes of heat transfer, namely conduction, convection, and radiation. Regardless of the mode of heat transfer by which thermal energy is transmitted to the environment, however, heat exchange apparatus of this kind is commonly referred to as a radiator. For this reason and for the additional reason that the present invention is concerned primarily with radiative heat exchange with the surrounding thermal environment, the heat exchanger of this invention, when used to dissipate heat, will be referred to in the ensuing description as a thermal radiator. It will become evident as the description proceeds, however, that the present radiator may utilize the conductive and/or the convective modes of heat rejection as well as the radiative mode.

In any thermal'control system of the kind under discussion, the amount of thermal energy which can be transferred from the system to be cooled, and hence the minimum temperature which can be maintained in the system, is a function of the tempert-ure of the thermal radiator employed in the thermal control system. This radiator temperature, or source temperature as it is commonly called, is, in turn, a function of the difference between the thermal energy absorbed by the radiator from the system being cooled and from the thermal environment in which the radiator operates and the thermal energy transferred from the radiator to the thermal environment. Accordingly, minimum temperature of the system to be cooled is obtained by miximiz-ing the theraml energy transfer from the radiator to its thermal environment and minimizing the thermal energy absorption by the radiator from the thermal environment. In a thermal control system, such as that of the present inventi-on, wherein radiation is the primary mode of heat transfer from the radiator to the thermal environment, maximum thermal energy transfer or radiation from the radiator is attained by exposing the external heat exchange surfaces of the radiator to a low temperature 3,285,333 Patented Nov. 15, 1966 heat sink in the thermal environment. In the case of the present radiator, this heat sink is typically a gaseous or v'acuous environment. Minimum thermal energy absorption by the radiator, on the other hand, is achieved, generally speaking, by making the radiator geometrically selective to the thermal radiation from its thermal environment, i.e., arranging the radiator to minimize or eliminate theincidence of environmental radiation on those radiator surfaces which are sensitive to such radiation, and/or making the radiator spectrally selective to the environmental radiation, i.e., rendering those surfaces of the radiator which receive environmental radiation insensitive to such radiation while retaining the radiation efficiency of the surfaces with respect to the thermal energy to be transmitted to the environmental heat sink.

Consider now a radiator situated above the ground and exposed to solar radiation, the radiator having external heat exchange surfaces adapted to radiate thermal energy to a heat sink above the radiator, and, therefore, sensitive to environmental thermal radiation. In this thermal environment, the radiator receives direct solar radiation, reflected solar radiation from the ground and thermal radiation emitted from the ground.

The radiator, to have maximum operational efficiency in such a thermal environment, must obviously be con structed in such a Way that it rejects both the direct and reflected solar radiation incident on all of the external radiator surfaces, rejects the environmental thermal radiation incident on all of the external radiator surfaces except its external, thermal-radiation-sensitive, he-at exchange surfaces, anad minimizes or eliminates the incidence of the environmental thermal radiation on these heat exchange surfaces. In the present invention, these requisites for maximum operational efficiency are satisfied by rendering the radiator both geometrically and spectrally selective to the environmental radiation in a new and unique manner.

According to the present invention, there is provided a thermal radiator comprising a thermally conductive radiator body including a set of first external surfaces which are oriented in one direction with respect to the radiator body and a set of second external surfaces which alternate with said first radiator surfaces and are oriented in another direction with respect to the radiator body. In the illustrative embodiments of the invention, for example, the radiator body is formed with external laterally directed fins having at one side, surfaces which face one end of the radiator body and comprise the first radiator surfaces mentioned above. The surfaces extending between the inner edges of these first fin surfaces and the outer edges of the adjacent fins, respectively, comprise the second radiator surfaces mentioned above.

Two specific thermal environments for which the present radiator is adapted have been selected for discussion herein to facilitate the description and a complete understanding of the invention. It will become evident as the description proceeds, however, that the present radiator maybe used in other thermal environments. Accordingly, it should be understood that the present radiator is not limited in application to the thermal environments disclosed herein for the sake of illustration.

' One illustrative thermal environment for the present radiator comprises simply a level surface exposed to solar radiation and above which the radiator is situated, as, for example, on the floor of a desert. According to the invention, a radiator intended for use in this thermal environment is so arranged that in its normal operative position, one of the two sets of external, differently oriented surfaces on the radiator body face upwardly away from the ground toward the sky as the heat sink above the radiator and are disposed in substantially horizontal, verti- 3 cally spaced planes. These upwardly facing surfaces function as heat exchange surfaces and are rendered spectrally selective to reject solar radiation and transmit thermal radiation. The thermal energy transferred from the system to be cooled is conducted to these surfaces and is radiated therefrom to the sky heat sink.

The remaining set of external surfaces on the radiator body, which alternate with these heat exchange surfaces, are made totally reflective to all radiation and are uniquely configured and oriented in such manner that a major portion of, or all, environmental radiation incident thereon from below the horizontal planes of the heat exchange surfaces, i.e., the thermal and reflected solar radiation from the desert floor, is reflected away from the underlying spectrally selective heat exchange surfaces. Thus, a major portion of or all thermal radiation from the ground surface, to which radiation the spectrally selective heat exchange surfaces are sensitive, is prevented from impinging the latter surfaces. Accordingly, the requirements for maximum heat transfer from the system being cooled, to wit, maximum thermal energy transfer from the radiator and minimum thermal energy absorption by the ra diator, are satisfied.

In a second illustrative thermal environment, the radiator is situated in a depression such as a gulch, canyon or valley having structures adjacent the radiator. In this case, the spectrally selective heat exchange surfaces of the radiator just discussed are inclined above the horizontal in such manner as to minimize or reduce to zero the view factor between the surfaces and the Walls of the depression or the adjoining structures. The thermal radiation transmitted from the depression walls or the adjoining structures to the thermal-radiation-sensitive exchange surfaces is thereby minimized or reduced to zero.

In the preferred forms of the thermal radiator disclosed herein, the reflective surfaces have a parabolic curvature in vertical section. These parabolic reflective surfaces are so oriented that substantially all thermal radiation from the surface of the ground or from structures adjoining the radiator is reflected beyond the outer edges of the underlying thermal-radiative-sensitive heat exchange surfaces of the radiator. As mentioned earlier and later more fully discussed, theforegoing thermal environments are intended to be purely illustrative and not limitative of the possible thermal environments in which the present radiator may operate. As also mentioned earlier and later discussed in more detail, a further important aspect of the invention is to provide a heat exchanger which is capable of use as a thermal energy absonber as well as a thermal energy radiator. In this connection, the invention provides a thermal energy absorber which discriminates between the environmental thermal and solar radiation and which is, therefore, useful, for example, in measuring one of these radiations to the exclusion of the other.

A general object of the present invention is, then, to provide a heat exchanger of novel configuration which is both geometrically and spectrally selective to incident radiation from its thermal environment.

Another object of the invention is to provide a heat ex-. changer which may be used to advantage either as a thermal energy radiator or as a thermal energy absorber.

The invention will now be described in detail by reference to the attached drawings wherein:

FIG. 1 illustrates one preferred embodiment of the present thermal radiator situated in a typical thermal environment for which it is designed;

FIG. 2 is an enlarged section through a portion of the Wall of the radiator in FIG. 1;

FIG. 2a is anenlarged wall section of a modified radiator according to the invention;

FIG. 3 is a perspective view of a simple cylindrical radiator;

FIG. 4 illustrates the radiator in FIG. 3 improved in accordance with this invention;

FIG. 5 is an enlarged section through a portion ofthe wall of the radiator in FIG. 4;

FIG. 6 is an enlarged wall section of yet a further modified radiator according to the invention;

FIG. 7 is an enlarged Wall section through a still further modified radiator according to the invention;

FIG. 8 illustrates one method of shielding the radiators to improve their efliciency;

FIGS. 9 anlO illustrate two alternative thermal environments for the radiator in FIG. 1; and

FIGS. 11 and 12 are enlarged wall sections'of thermal energy absorbers according to the invention.

Referring first to FIG. 1 of these ,drawings, there is illustrated a thermal radiator 10 according to the invention situated on level ground 12. Radiator 10 receives direct solar radiation S solar radiation S, reflected from the surface 12, and thermal radiation T emitted by the surface 12 as a result of heating of the surface by the solar radiation S As will be seen shortly, the radiator 10 radiates thermal energy T to the sky.

The heat generating system or heat source 14 to be cooled by radiator 10 has, for convenience, been illus trated as not enclosed by the radiator. Thermal energy generated by this system is transferred, via heat exchange means (not shown), from the system 14 to a fluid coolant which circulate-s through tubes 16. As may be best observed in FIG. 2, tubes 16 are disposed in heat transfer relation to the thermally conductive side wall 18 of the I radiator 10. This wall, in .a typical radiator, will be cylindrical. It will become evident as the description proceeds, however, that the invention is not limited to a' radiator of such cylindrical shape.

Referring now particularly to FIG. 2, the radiator wall 18 has an inner surface 20 to which the coolant tubes 16 are bonded, in any conventional manner, so as to pro vide efiective heat transfer from the fluid coolant through the tubes to the radiator wall. The outer surface of the I radiator wall 18 is formed with a series of circumferential grooves 22 defining fins 23. These fins have, at one side,

axially facing surfaces 24. Extending between the inner edge of each fin surface 24 and the outer edge of the adjacent fin is a generally radially facing surface 26. In the form of the invention under discussion, the axially facing surfaces 24 are disposed in planes P substantially normal to and spaced along the central axis 28 of the radiator 10. The'intervening radially facing surfaces 26 are preferably, though not necessarily, of concave curvature in transverse section, as shown, for reasons which will appear as the description proceeds. These axially and radially facing surfaces extendaround the entire circumference of the radiator wall 18. Grooves 22 which define the surfaces 24 and 26 may be formed by any convenient process, such as milling, grinding, chemical etching, rollmg, etc. J

The axially facing radiator surfaces 24 are renderedspectrally selective in some known way, such as by applying thereto a film or layer 30 of spectrally selective paint or some other suitable spectrally selective material having a relatively low absorptivity over the range of the high temperature solar radiation S and a relatively high emissivity over the range of the low temperature thermal radiation T to be dissipated from the radiator. The intervening radially facing radiator surfaces 26 are rendered totally reflective in any conventional way, such as by applying thereto a film or layer 32 of polished silver or other reflective material having a relatively low absorptivity over. the entire thermal energy radiation spectrum. As will appear shortly, the axially facing radiator surfaces 24 withtheir spectrally selective coating 30 form the thermallyco'nductive heat exchange surfaces of the radiator 10. In the ensuing description, these surfaces will be referred to as heat exchange surfaces 30 or simply as heat exchange surfaces. The intervening surfaces 26- with their reflective coating 32 form reflecting surfaces and will be hereinafter referred to as reflecting surfaces 32 or simply as reflecting surfaces.

When situated in the thermal envoriment of FIG. 1, radiator may be supported on a structure 34 some distance above the surface 12 with the radiator axes 28 vertically oriented. Under these conditions, the direct solar radiation 8., arrives at the radiator from above or parallel to the horizontal plane-s P of the heat exchange surfaces 30. The reflected solar radiator S and environmental thermal radiation T arrive at the radiator from below the planes P. A portion of the thermal energy contained in the fluid coolant circulated through the tubes 16 is transferred, primarily by conduction, to the radiator wall 18 and a portion of this transferred thermal energy is, in turn, transmitted upwardly to the heat sink from the heat exchange surfaces 30 as thermal radiation T During operation, the equilibrium temperature of the radiator 10 is a function of the difference between the thermal radiation T transmitted from the radiator surfaces 30 and the total direct solar, reflected solar, and environmental thermal radiation S S and T absorbed by all external surfaces of the radiator as well as the thermal energy transferred from. the system being cooled to the radiator. The present invention is concerned primarily with the novel geometry of the radiator surfaces 30' and 32 whereby the transmitted radiation T exceeds the total absorbed radiation 8, 8,. and T sufficiently to maintain a minimum radiator temperature and thereby maintain a minimum temperature in the system 14 to be cooled.

Before proceeding with a further discussion of this novel radiator surface geometry, reference is made to FIG. 3 which illustrates a simple cylindrical thermal radiator situated in the thermal environment of FIG. 1. In FIG. 3, the radiator R has a nonspectrally selective radiator surface S exposed to the total environmental radiation S S and T This radiator obviously would, at best, be highly inefficient and very probably completely inoperative in the thermal environment. The efliciency of this simple radiator configuration can be improved somewhat by coating its surface S with a spectrally selective material designed to reject the incident solar radiation 8,, and 8,. The spectrally selective heat exchange surface thus provided, however, is sensitive toand absorbs the thermal radiation T from the surface 12. Such a radiator, then is only spectrally selective to its thermal environment.

According to one aspect of the present invention, the efliciency of the cylindrical radiator of FIG. 3 is further improved by providing it with heat exchange fins F (FIGS. 4 and 5) which are horizontally oriented such that only direct solar S impinges the upper surfaces while reflected solar radiation S and environmental thermal radiation T impinge the lower fin surfaces. Then, by coating the upper fin surfaces with a spectrally selective material M (FIG. 5) designed to reject solar radiation and conduct or radiate (and thereby also absorb) thermal radiation and coating the lower fin surfaces and the cylindrical radiator surface between the fins F with a reflective material M designed to reflect all radiation, the total radiation absorbed by the radiator can be minimized while the radiation efliciency of the upper heat exchange surfaces of the radiator fins is preserved.

It is obvious from the foregoing discussion that in the case of the radiator of FIG. 4, the radiation to which the upper heat exchange surfaces of the fins are sensitive, namely the environmental thermal radiation emanates from below the horizontal planes of these surfaces. Accordingly, the heat exchange surfaces are geometrically oriented to have a zero view factor with respect to the source of the environmental thermal radiation to which they are sensitive and have a spectral selectivity which renders the surfaces relatively insensitive to the direct solar radiation incident thereon, Further, as illustrated in the upper part of FIG. 5, some reflected solar radiation S and environmental thermal radiation T arriving at the radiator R from distant point sources is reflected from the cylindrical radiator surface, to the underside of each upper fin F and then away from the radiator, as well as from the under surface of each fin F, to the cylindrical radiator surface, and then away from the radiator, without impinging the upper thermal-radiation-sensitive surfaces M of the fins. The radiator configuration of FIGS;

4 and 5 is, in other words, both geometrically and spec-' trally selective with respect to its thermal environment.

This simple radiator configuration is, however, deficient in that environmental thermal radiation T and reflected solar radiation S from point sources close to the radiator is reflected from the under surface of each fin to the cylindrical radiator surface and then to the upper thermalradiation-sensitive surface M on the next lower fin where it is absorbed, as indicated in the lower part of FIG. 5. Accordingly, the finned radiator configuration of FIGS. 4 and 5 is only partially geometrically selective to its thermal environment.

According to a further aspect of the present invention, this deficiency of the simple thermal radiator configuration of the invention shown in FIGS. 4 and 5 is avoided by a unique geometry and orientation of the heat exchange surfaces 30 and the reflecting surfaces 32 of the preferred radiator configuration shown in FIGS. 1 and 2 whereby a major portion or all radiation incident on the reflecting surfaces from below the horizontal planes P of the heat exchange surfaces 30, including, therefore, the thermal radiation from the ground 12 to which the heat exchange surfaces are sensitive, is reflected away from the latter surfaces. In the preferred form of the invention, this aim of the invention is accomplished by providing the reflecting surfaces 32 with a curvature parabolic in transverse section, that is in planes containing the radiator axis 28. Each reflecting surface 32 is, then, a semi-parabolic surface of revolution. The axis A of each circumferential increment of each parabolically curbed reflecting surface 32 is preferably contained in the horizontalplane P of the immediately underlying heat transfer surface 30. The focal point 7 of each such increment is preferably located at the outermost edge of the immediately underlying heat transfer surface.

With the foregoing description of the thermal radiator 10 and the thermal environment illustrated in FIG. 1 in mind, and assuming that the upper end of the radiator is shielded in some manner, such as by an upper structure or a reflective coating (not shown), consider first the solar radiation incident on the radiator. All direct solar radiation 8,, incident on the radiator impinges the spectrally selective, heat exchange surfaces 30 and/ or the reflecting surfaces 32 depending upon the solar angle, that is the angle 0 (FIG. 1) between the radiator axis 28 and the direct solar radiation vector. The spectrally selective coating comprising the heat exchange surfaces 30, being relatively insensitive or non-conductive to solar radiation, rejects or reflects a high percentage of the direct solar radiation arriving at the latter surfaces. This direct solar radiation is reflected away from the radiator by secondary reflection from the reflecting surfaces 32. Similarly, a high percentage of the direct solar radiation arriving at the reflecting surfaces 32 is reflected away from the radiator either directly or by secondary reflection from the spectrally selective heat transfer surfaces 30. Thus, regardless of the solar angle, a high percentage of all direct solar radiation S incident on the radiator 10 from its thermal environment in FIG. 1 is reflected back to the environment, as shown in the upper part of FIG. 2.

Consider next the reflected solar radiation S, and the environmental thermal radiation T from the surface 12. The heat exchange surfaces 30 being horizontal and parallel to the surface 12 have a zero view factor with respect to the surface, regardless of the horizontal distance of a point source of radiation on the surface 12 from the radiator. Accordingly, all reflected solar and thermal radiation incident on the radiator 10 is transmitted to the latter from below the horizontal planes P of the heat transfer surfaces 30 and the axes A of the parabolically curved radiator reflecting surfaces 32.

Recalling the characteristics of a parabolic reflector, it is evident that both the reflected solar and thermal radiation from the surface 12, arriving as it does, at the radiator-"10 from below the planes of the axes of the parabolic reflecting surfaces, is reflected from the latter surfaces in such manner that substantially no reflected solar or environmental thermal radiation impinges the heat exchange surfaces 30. This can be demonstrated by considering one of the reflecting surfaces 32 of the radiator 10, say the lower surface 32 in FIG. 2, and selecting an incremental area of dA of this surface. Assume now a'point source of thermal radiation T,, on the surface 12 which can be moved toward and away from the radiator along the intersection of the latter surface with the vertical or radial plane containing the axis 28 ofthe radiator and the area dA. If we move the point source toward the radiator, a limiting position is reached in which thermal radiation is transmitted from the source to the surf-ace area dA along a direction line I crossing the outer edge of the adjacent lower heat transfer surface 30 and passing approximately through the parabolic focal point 1 located at the latter edge. This radiation is reflected from the area dA along the direction line II which approximately parallels the respective parabolic axis A. If the point source is moved closer to the radiator, the outlet edge of the lower heat transfer surface 30 obscures the source from the view of surface area dA.

Assume now that we move the point source of thermal radiation away from the radiator 10. As the point source recedes from the radiator, the angle 5 between the direction line I of the thermal radiation incident on the area dA from the source and the horizontal plane P of the lower parabolic axis A approaches zero. The direction line II of the reflected radiation from the surface area dA, on the other hand, is rotated, in effect, in the clockwise direction about the area dA as'a center and toward the focal point 1. The limiting value of the angle is zero, of course, regardless of the distance between the radiator and the point source. Under this latter limiting condition, the incident direction line 1' becomes parallel to the lower plane P and the reflection direction line 11 passes through the focal point 7 which is located at the outer edges of the lower heat transfer surface 30.

It is evident from this discussion that all environmental thermal radiation arriving at the surface area dA from point sources located on the ground 12 in the radial plane under consideration is reflected away from the radiator in such manner that no thermal radiation from .these sources impinges the underlying surface 30. The reflected solar radiation S, from the latter sources is also reflected away from the lower heat exchange surface, of course. This reflection of the reflected solar radiation, however, is not critical since the heat exchange surfaces reject the reflected as well as the direct solar radiation.

It is evident that the above analysis is valid for each increment of each reflecting surface of the radiator and for every radial plane of the radiator. It is further evident, of course, that each incremental reflecting surface area dA receives environmental thermal radiation in oblique directions, that is directions inclined to the radial plane containing the respective area dA. It is evident however, that such oblique thermal radiation is also reflected away from the thermal energy-sensitive heat exchange surfaces 30.

From the foregoing description, it is clear that the heat exchange surfaces 30 of the radiator 10 rejectthe solar radiation incident thereon and radiate, to the heat sink above the radiator, the thermal energy transferred to the radiator from the system 14 to be cooled. The reflecting surfaces 32 of the radiator, on the other hand, reflect all incident thermal and reflected solar radiation from the ground 12 away from the thermal-radiation-sensitive heat exchange surface. The radiator is, therefore, fully spectrally and geometrically selective to the particular thermal environment illustrated in FIG. 1.

Other reflecting surface geometries than parabolic may be utilized on'the radiator. Thus, the reflecting surfaces 32 may comprise any conic section. For example, the reflecting surfaces could be conical, as shown at 32a in FIG. 2a, in which case the intersection of a reflecting surface with a radial plane of the radiator would be a straight line. Such conical reflecting surfaces, however, are effective to completely reflect away from the spectrally selective heat exchange surfaces 30a only that environmental thermal radiation arriving at the reflecting surfaces 32a along direction linesinclined to the horizontal 'by angles which are equal to or greater than the angle between the horizontal and the normals N to the reflecting surfaces.

In other words, the radiator configuration of FIG. 2a is sensitive to environmental thermal radiation from point sources relatively distant from the radiator but is insensitive to thermal radiation from point sources relatively close to the radiator. In this respect, the radiator configuration of FIG. 2a is superior to that of FIGS. 4 and 5 which is insensitive to thermal radiation from point sources relatively close to the radiator and insensitive to thermal radiation from point sources remote from the radiator.

Reflecting surfaces with curvature circular in transverse cross-section rather than parabolic could also be utilized. Because of spherical aberration, however, some environmental thermal radiation will be reflected to the sensitive heat exchange surfaces if circularly curved reflecting surfaces are used.

Accordingly, neither conically nor circularly curved reflection surfaces afford the radiator 10 with the same degree of geometrical selectivity to the thermal environment of FIG. 1 as do the parabolically curved reflecting surfaces 1 illustrated in FIG. 2. Other conic section reflecting surface geometries are, of course, possible. The invention should not, therefore, be regarded as limited to the parabolic reflecting surface geometry illustrated.

The description thus far has been directed toward thermal radiator geometries which minimize or eliminate the incidence of thermal radiation on the thermal-radiation-sensitive heat exchange surfaces of the radiator from .a thermal environment of the kind illustrated in FIG. 1, Le. one in which all environmental thermal radiation originates from below the horizontal planes of the heat transfer surfaces. Assume, however, that the radiator 10 of FIG. 1 is placed in the thermal environment of FIG. 6 in which the surface of the ground 100 has a slope 102, as in an open pit mine or in a canyon.

When situated in this thermal environment, the horizontal heat transfer surfaces of the radiator 10 would obviously view and, therefore, receive thermal radiation from the slope 102. A similar thermal environment would be created by a'building structure or other structure situated on the ground adjacent the radiator. According to the present invention, the view factor of the thermal-radiation-sensitive heat exchange surfaces of the radiator with respect to the source of such radiation, i.e.,, the slope 102 or a building structure adjacent the radiator, is reduced to zero by providing a radiator 104 having spectrally se-' lective heat exchange surfaces 106 which are inclined to the horizontal, as shown. In the case of the thermal environment of FIG. 6, the angle between the horizontal and the heat exchange surfaces 106 is preferably equal to or greater than the slope 102. Thus, direct impingement of thermal radiation from the slope on the heat exchange surfaces is avoided or greatly minimized.

The radiator 104 illustrated'has total reflecting surfaces 108 which are preferably parabolically curved like those on the radiator 10 and which serve the same purpose as the reflecting surfaces on" the radiator 10. In the case of the radiator 104, however, the axesA of the parabolic surfaces, being coplanar with the radiator surfaces 106,

are inclined above the horizontal. The heat exchange surfaces of the other disclosed radiator configurations can be similarly inclined, of course.

In operation, the thermal radiator 104 functions in precisely the same way as radiator 10. Accordingly, no further discussion of radiator 104 is deemed necessary.

It is well known, of course, that no reflecting or spectrally selective surface possesses zero absorptivity. Accordingly, in all of the present radiator configurations thus far described, a small percentage of the direct and reflected solar radiation and environmental thermal radiation incident on both the spectrally selective heat exchange surfaces and the reflecting surfaces of the radiators is absorbed and produces heating of the radiator. If we define the solar angle as the angle between the radiator axis, i.e. axis 28 in FIG. 1, and the solar radiation vector, whereby 0=0 represents solar noon,

=90 represents solar sunrise,

0=90 represents solar sunset, and

0=180 represents solar midnight,

and the radiator angle as the angle, measured around the radiator, between a given incremental surface area of the radiator and the plane containing the radiator axis and the solar radiation vector, the solar radiation absorbed by each incremental surface area of the radiator, and hence the total solar radiation absorbed by the radiator, varies as a function of both the solar angle and the radiator angle. The thermal energy radiated from each incremental area of the heat exchange surfaces also varies, of course, with changes in the solar angle and the radiator angle.

It is evident, therefore, that the effective temperature of the present thermal radiator, at any given solar angle, varies around the radiator and that the effective temperature of a given circumferential portion of the radiator changes as the solar angle changes. At any given solar angle, other than 0 and 180, the minimum temperature exists at a radiator angle of 180, the temperature progressively increasing in either direction around the radiator to a maximum at a radiator angle of 0. At solar angles of 0 and 180, the temperatures are uniform about the radiator. The temperature at any given radiator angle attains a maximum at solar angles between 45 and 90, since at these angles, both the radiator heat exchange and reflecting surfaces receive, and therefore absorb, maximum direct solar radiation. At smaller solar angles, these surfaces are partially or completely shielded against direct solar radiation. During a complete solar day, then, a maximum heat temperature occurs at a radiator angle of 0 and solar angles of 45 and 90.

Mathematical heat balance analysis of the present radiator has established'the fact that the above maximum temperature of the radiator may be decreased by providing the radiator with additional circumferential, spectrally selective heat exchange surfaces between adjacent heat exchange surfaces as indicated at 200 in FIG. 7. Surfaces 200 comprise a material which, like the spectrally selective heat exchange surfaces 30 described earlier, reject solar radiation but transmits (and therefore also absorbs) thermal radiation. Radiator surfaces 200 are, of course, exposed to and therefore absorb thermal radiation from the ground above which the radiator is located. As a result, the reduction in the maximum radiator temperature is obtained by the radiator geometry in FIG. 7 at the cost of an increase in the minimum radiator temperature. In general, then, increasing the area of the circumferential radiator surfaces 200 reduces the maximum temperature, which occurs in the portion of the radiator subjected to the most critical solar radiation conditions, and increases the minimum temperatures which occurs in the portion of the radiator subjected to the least critical solar radiation conditions.

It is obvious from the description thus far that a thermal radiator constructed in accordance with this invention has, at any given solar angle other than 0 and 180, a relatively hot area facing toward the direct solar radiation and a relatively cool area facing away from such radiation. It is obvious, therefore, that the effective temperature of the radiator. can be further minimized if only the relatively cool radiator area is utilized. This can be accomplished, for example, by employing a shield 300 (FIG. 8) which is rotatable around the radiator to constantly shield the radiator area exposed .to the most direct solar radiation. Alternatively, the flow system 16 (FIG. 1) for recirculating the fluid coolant through the radiator may incorporate valves 400 which can be selectively operated to direct the fluid coolant through only the cool portion of the radiator.

As mentioned earlier, the particular thermal environments, and the particular orientation of the radiators with respect to the radiation vectors of these environments, disclosed herein, are intended to be purely illustrative and not limiting in nature. In other words, the present thermal radiator geometry can be used to advantage in thermal environments other than those specifically disclosed herein and can be oriented differently with respect to the radiation vectors of the environments herein described. Depending upon the particular thermal environment and radiator orientation involved, one or the other of the sets of differently oriented radiator surfaces 24 and 26 may be leftuncoated, i.e., without the film-30 or 32, and/or the location of these films may be reversed.

For example, the radiators described earlier might be required to operate only during the night hours, whereby solar radiation would be absent. In this case, the radiator surfaces 24 may be left bare, i.e., without the spectrally selective film 30, while the parabolic reflecting surfaces 32 would be retained to reflect the incident environmental thermal radiation existing due to heating of the surface of the ground during the daylight hours, as illustrated in FIG. 9.

Another possible thermal environment is that shown in FIG. 10 wherein the environmental thermal radiation is negligible. In this case, the radiator is oriented so that the radiator surfaces 24 face away from the solar radiation source and the parabolic radiator surfaces 26 receive the incident solar radiation throughout solar angles between 0 and In this situation, the parabolic radiator surfaces 26 are rendered spectrally selective or totally reflected by a film 30 (or 32) to reject the incident solar radiation while the radiator surfaces 24 are left bare to serve as heat exchange surfaces along with the surfaces 26 if the latter are rendered only spectrally selective. It is obvious that in this latter mode of operation, the parabolic surfaces 26 are eifeotive to reflect the incident solar radiation away from the bare heat exchange surfaces 24.

As mentioned earlier, a heat exchanger constructed in accordance with this invention may also be used as a geometrically spectrally selective thermal energy absorber. Referring to FIG. 11, for example, .there is shown a thermal energy absorber 500 situated in the thermal en vironment of FIG. 1. This energy absorber is identical to the radiator of FIG. 1 except that the transverse heat exchange surfaces 24 of the absorber are left bare so that they absorb the incident solar radiation S The parabolic surfaces 32 of the absorber are totally reflective, as in the radiator of FIG. 1, so that reflected solar and environmental thermal radiation S and T is reflected away from the surfaces 24. Such an absorber, then, is sensitive to only direct solar radiation and could be used, for example, in a system for measuring such radiation to the exclusion of the environmental thermal radiation and reflected solar radiation.

The thermal energy absorber 600 of FIG. 12 is identical to that of FIG. 11 except that the absorber of FIG. 12 is inverted. In this case, the reflective parabolic surfaces 32 of the absorber reflect the direct solar radiation while 1 l the bare surfaces 24 absorb the environmental thermal radiation T and the reflected solar radiation 8,. This latter absorber, then, is sensitive to only environmental thermal and reflected solar radiation.

'It will be immediately evident that any of the thermal radiators described earlier can be used, as they stand, as thermal energy absorbers which are sensitive to environmental thermal radiation only. In other words, if the radiator in FIG. 1, for example, is inverted, its then lower spectrally selective surfaces will absorb the environmental thermal radiation while both the direct solar and reflected solar radiation will be rejected by the absorber. Such a heat exchanger or thermal energy absorber, then, could be used to measure the environmental thermal radiation to the exclusion of both the directand reflected solar radiation.

Other modes of operation of both the present thermal energy absorbers and the thermal energy radiators will present themselves to those skilled in the art.

Clearly, therefore, the invention herein described and illustrated 'is fully capable of attaining the objects and advantages preliminarily set forth.

Numerous modifications in the design, arrangement of par-ts, and instrumentalities of the invention are possible Within its spirit and scope.

I claim:

A 1. A heat exchanger, comprising:

a thermally conductive body having a set of first exposed surfaces oriented in one direction relative to said body and a set of second exposed surfaces a1- ternating with said first surfaces and oriented in another direction with respect to said body; and

one of said sets of surfaces being spectrally selective and having relatively 'low absorptivity over the range of high temperature solar radiation only and the other set of surfaces being substantially to tally reflective over the entire range from relatively low temperature thermal radiation to high temperature solar radiation.

2. A heat exchanger comprising:

a thermally conductive body including exposed, generally parallel, laterally directed fins;

said fins having first surfaces, respectively, at one side and therebeing a second surface extending between the inner edge of each fin surface and the outer edge of the adjacent fin, whereby said body has a set of exposed first surfaces oriented in one direction relative to the body and a set of exposed second surfaces alternating with said first surfaces and oriented in another direction relative to the body; and

one of said sets of surfaces being spectrally selective v and having relatively low absorptivity over the range of high temperature solar radiation only and the other set of surfaces being substantially totally reflective over the entire range from relatively low temperature thermal radiation to high temperature solar radiation. p

3. .The subject matter of claim 2 wherein: said first surfaces are spectrally selective and said second surfaces are substantially totally reflective.

4. A thermal radiator comprising:

a cylindrical, thermally conductive radiator body including a multiplicity of external, circumferentially extending, radially directed fins;

said fins having first surfaces, respectively, at one side presented toward one end of said body and there being a second parabolically curved surface extendingj between the inner edge of each first finsurface and the outer edge of the adjacent fin, whereby said body has a set of first surfaces oriented in one direction relative to the body and a set of second surfaces alternating with said first surfaces and oriented in another direction relative to the body; and

said sets of surfaces having different thermal absor tivities.

5. The subject matter of claim 4 wherein:

said first surfaces are spectrally selective and have relatively low absorptivity over the range of high temperature solar radiation and relatively high emissivity over the range of low temperature thermal radiation, and said second surfaces are substantially totally reflective.

6. A thermal energy absorber, comprising:

a cylindrical, thermally conductive body including a multiplicity of external, circumferentially extending, radially directed fins;

said fins having first surfaces, respectively, at one side presented toward one end of said body and there being a second, parabolically curved surface extending between the inner edge of each first fin surface and the outer edge of the adjacent fin whereby said body has a set of first surfaces oriented in one direction relative to the body and a set of second surfaces alternating with said first surfaces and oriented in another direction relative to the body; and

said sets of surfaces having different thermal absorptivities.

7. The subject matter of claim 6 wherein:

said first surfaces are spectrally selective and have relatively low absorptivity over the range of high temperature solar radiation and said second surfaces are substantially totally reflective.

8. For use in a heat exchanger:

a thermally conductive body including exposed, generally parallel, laterally directed fins;

said fins having first surfaces at one side and there being a second surface extending between the inner edge of each first fin surface and the outer edge of the adjacent fin, whereby said body has a set of first exposed surfaces oriented in one direction relative to said body and a set of second exposed surfaces alternating with said first surfaces and oriented in another direction relative to said body;

the intersections of said second surfaces with a plane References Cited by the Examiner UNITED STATES PATENTS 2,372,155 3/1945 Bosch l65l33 X 2,914,915 12/1959 Sziklas'et al. -133 X 3,045,138 7/1962 POhl 165l80 X FOREIGN PATENTS 115,481 7/ 1942 Australia. 387,828 2/1933 Great Britain.

ROBERT A. OLEARY, Primary Examiner.

CHARLES SUKALO, Examiner.

A. ANTO A stant Examiner. 

1. A HEAT EXCHANGER, COMPRISING: A THERMALLY CONDUCTIVE BODY HAVING A SET OF FIRST EXPOSED SURFACES ORIENTED IN ONE DIRECTION RELATIVE TO SAID BODY AND A SET OF SECOND EXPOSED SURFACES ALTERNATING WITH SAID FIRST SURFACES AND ORIENTED IN ANOTHER DIRECTION WITH RESPECT TO SAID BODY; AND ONE OF SAID SETS OF SURFACES BEING SPECTRALLY SELECTIVE AND HAVING RELATIVELY LOW ABSORPTIVITY OVER THE RANGE OF HIGH TEMPERATURE SOLAR RADIATION ONLY AND THE OTHER SET OF SURFACES BEING SUBSTANTIALLY TO TALLY REFLECTIVE OVER THE ENTIRE RANGE FROM RELATIVELY LOW TEMPERATURE THERMAL RADIATION TO HIGH TEMPERATURE SOLAR RADIATION. 